1. Field of the Invention
The present invention relates to an improvement in an apparatus for detecting the desired or reasonable condition or ratio of developer in an electrostatic photographic copying machine, wherein changes in magnetic characteristics of the developer is detected by a magnetic sensor and the desired ratio of the developer is maintained on the basis of the detected result.
2. Description of the Prior Art
In a copying machine in which the developer consists of insulating substances or particles which are consumed and magnetic substances or particles, it is important to supply new insulating particles upon the consumption thereof to maintain the concentration of insulating particles within the desired ratio. Also, when the developer consists of only one component, it is important to supply new developer upon the consumption thereof to maintain the level thereof within a certain reasonable or desired condition or range.
In general, a coil has been used as the magnetic sensor for maintaining the reasonable condition of the developer, a change in the magnetic permeability of the developer being detected from a change in inductance of the coil thereby detecting the reasonable condition, i.e. the ratio or level, of the developer. In such case, it is necessary for the developer to flow over the surface of the magnetic sensor, and the conventional apparatus has the following disadvantages.
According to the conventional apparatus, as shown in FIG. 4, a magnetic sensor 15 is embedded in a fitting member 16, and a gap 18, though small, unavoidably is produced between magnetic sensor 15 and fitting member 16. As a result, developer 9 is introduced into gap 18 and adheres therein. The adhered developer 9 is gradually accumulated due to magnetic action thereof until at least a part of magnetic sensor 15 is covered with such developer 9. In particular, in the case, as shown in FIG. 4, when sensor 15 is arranged inwardly from a surface F of fitting member 16 to produce a step between the surface of sensor 15 and surface F of fitting member 16, developer 9 is even more apt to adhere to the surface of magnetic sensor 15 due to such step. This conventional apparatus has the disadvantage that the adhesion of developer 9 to the surface of magnetic sensor 15 leads to an insufficiently smooth flow of developer 9 on the surface of magnetic sensor 15, as a result of which the sensor is incapable of detecting the actual condition of the developer.